The present invention relates to a method for the isomerization of a cis-alkenol compound to a corresponding trans-alkenol compound.
In recent years, there has been proposed a promising method for the extermination of noxious insects such as those belonging to the Lepidoptera, e.g. butterflies and moths, utilizing a so-called sexual pheromone compound which is excreted by a particular insect of a sex and attracts very strongly the insects of the same species of another sex even in an extremely low concentration. Many of the sexual pheromone compounds of the Lepidoptera has a chemical structure in which one or two of the ethylenic double bonds are included in a straight chain molecule. Most of these compounds are the cis-isomers in the geometrical isomerism relative to the double bonds but some of the sexual pheromone compounds are a mixture of the cis- and the trans-isomers.
For example, the sexual pheromone compound of nashihimeshinkuiga (oriental fruit moth) is a mixture of the cis- and the trans-isomers of 9-dodecenyl acetate in a proportion of 93:7. Further, the sexual pheromone compound of tsuzurimonhamaki is a 1:1 mixture of cis- and trans-11-tetradecenyl acetates.
Usually, there is no particular problem in the synthetic preparation of the cis-isomers of the above described type since catalytic partial hydrogenation of an acetylenically unsaturated compound gives the cis-isomer of the corresponding ethylenically unsaturated compound in a much larger yield than the trans-isomer. When a mixture of the cis- and the trans-isomers of an ethylenically unsaturated compound is desired, however, no industrially advantageous method is known to obtain such a mixture with a desired ratio of the trans-isomer to the cis-isomer since the content of the trans-isomer in the mixture can rarely exceed 2 to 5% even under the most favorable conditions for the formation of the trans-isomer. When a higher content of the trans-isomer is desired relative to the cis-isomer, therefore, it is a usual practice that the trans-isomer is synthesized separately and mixed with the cis-isomer in a desired proportion although such a method is industrially not practicable since no advantageous method is known for the synthesis of the trans-isomer alone.
Thus, it has long been desired to develop an industrially advantageous method in which relatively easily available cis-isomers are isomerized into the corresponding trans-isomers in a simple process.